For producing such thin film magnetic head, upper and lower magnetic layers of Sendust or amorphous materials, coil conductor layers and insulating layers, interposed between the magnetic layers, are formed on a substrate formed of a wear-resistant material, such as ferrite or sapphire through repeated layer forming and etching for patterning to a predetermined shape. And ultimately a protective layer is formed to protect the magnetic layer from abrasion caused by the contact running of the magnetic recording medium. Alumina or SiO.sub.2 has been used as the material for the protective layer.
Meanwhile, it is well-known that, if the protective layer is sufficiently harder than the magnetic layer, the magnetic layers undergo wear at an earlier time through the contact running of the recording medium to produce partially advancing abrasive wear, referred to as partial abrasion, in the magnetic layers, and hence the spacing loss.
On the other hand, if the protective layer is too soft, wear is promoted on the whole so that the service life of the head is shortened. It is therefore preferred that the protective layer has a hardness about equal to or slightly lower than that of the magnetic layers.
For example, with the Vickers hardness Hv of the magnetic layers of 600 to 650 kgf/mm.sup.2, the vickers hardness Hv of the protective layer is preferably set so as to be in the range of from 400 to 600 kgf/mm.sup.2.
It is also necessary for the thickness of the protective layer to be about 20 to 40 .mu.m or more in view of sliding properties and the resistance to partial abrasion of the recording medium. However, with this order of the layer thickness, the protective layer usually undergoes peeling or cracking under the accumulated internal stress. It is therefore necessary to reduce the internal stress to as low a value as possible. One of efficient measures to cope with this problem is to match the thermal expansion coefficients of the respective component materials of the magnetic head with one another. However, it has been difficult in general to adjust the thermal expansion coefficient of the magnetic metal material to that of the protective layer.
The JP Patent Kokai Publication 62-16218 discloses a mixture of MgO and SiO.sub.2 as a material for the protective layer which is effective to prevent the occurrence of partial abrasion. It is also disclosed therein that, by setting the MgO/SiO.sub.2 compositional ratio so as to be 10 to 70% in terms of SiO.sub.2 amount, a moderate hardness Hv of 450 to 850 and a desirable thermal expansion coefficient may be achieved.
However, it has not been possible with this protective layer to satisfy the requirements for corrosion resistance or workability in addition to the requirements concerning the basic properties of the protective layer.